The present invention relates to an improved efficiency of radio frequency power amplifiers for use in portable cellular phone terminals and base stations incorporated in a mobile communication system.
Recently, in the field of the mobile communication, scaling down in the size and weight is requested not only for the portable cellular terminals but also for the base stations. Along with this trend is further requested the improvement of the efficiency of the power amplifiers, which affects mostly the electric consumption of the whole system. The portable cellular phone terminal, which usually use a battery for their main power source, may have a major problem that requires to improve the efficiency of the phone terminal, by saving the power consumption in the power amplifier unit which drains the most power in the circuitry, in order for the terminal to sustain its operating time. In addition, since the radio frequency power amplifier are installed near their antenna for the objective of decreasing some transmission line loss in the cable and the like, shrinking the size and improving the efficiency of radio frequency power amplifier is highly required.
In general, the efficiency of an radio frequency power amplifier using semiconductor elements will be increasingly higher if the output power is higher, and will be highest at the yieldable maximum output, i.e., in vicinity of the saturated output power level. Also, the saturated output power level may be dependent on the size of semiconductor elements used.
When making a power amplifier of low saturated output power level by using smaller semiconductor elements in order to improve the efficiency at lower output power, output power required may not be sufficient at higher output power state. On contrary, when making a power amplifier of higher efficiency at some high saturated output power level using larger semiconductor elements, the efficiency will be decreased at lower output level.
It can be seen from the foregoing by those skilled in the art that it is quite difficult to operate one single power amplifier at high efficiency in both lower and higher output power mode. As a solution it has been proposed, as disclosed for example as a circuit embodiment in Japanese Patent Laid-open (Kokai) No. Hei 7-336168, a circuit arrangement which may be able to yield an improved efficiency at both higher and lower output power modes, by providing a plurality of output stage power amplifiers each having a different saturation output power level and by switching to one most convenable output stage power amplifier in accordance with the output power, by means of a switch.
A typical example of conventional power amplifier capable of switching output power stage in accordance with the desired output level is shown in FIG. 16. In the figure, an incoming radio frequency signal that have been received by input terminal IN1 will be amplified in high output power amplifier component A161 designed to be used for high output power level. An amplified signal then will be passed through a matching network M161 to a radio frequency switch SW1 used for switching the signal path. Meanwhile a radio frequency signal incoming via input terminal IN2 will be amplified by low output power amplifier component A162 designed to be used for low output power level, and will be passed through another matching network M162 to the radio frequency switch SW1. The radio frequency switch SW1 will be turned to the high output power amplifier component A161 for high output power mode (in which case the low output power amplifier component A162 will be put into stop of operation state), or will be turned to the low output power amplifier component A162 if lower output power is required. (in which case the high output power amplifier component A161 will be put into stop of operation state). Thus through output terminal OUT will be delivered output a signal amplified either one of output power amplifier elements.